1. Field of the Invention
The present invention relates to the field of alternative energy sources and means for extracting energy from those sources. More particularly, the present invention relates to biological and biochemical degradation of plant material, including lignocellulose, for production of energy for use in human activities.
2. Description of Related Art
Production of biological-based products and bio-energy from renewable lignocellulose is of importance to sustainable development of human industrial society in the face of the depletion of natural resources, especially fossil fuels, and the resulting accumulation of atmospheric carbon dioxide (CO2). Also, development of technologies for effectively converting agricultural and forestry residues to fermentable sugars offers outstanding potential to benefit the U.S. national interest. Furthermore, production of these second generation biofuels, such as cellulosic ethanol from renewable lignocellulosic biomass, as well as third generation biofuels, such as hydrogen and electricity, will lead the bioindustrial revolution necessary to the transition from a fossil fuel-based economy to a sustainable carbohydrate economy. Use of biofuels will offer several benefits, including reduced greenhouse gas emissions, decreased competition with tightening food supplies, enhanced rural economic development, and increased national energy security. However, key technological challenges in this area include finding new technologies for energy production and reducing the cost of technologies for converting biomass (primarily lignocellulose) into desired biobased industrial products and bioenergy.
Lignocellulosic biomass, such as agricultural and forestry residues, municipal and industrial solid wastes, and herbaceous and woody bioenergy plants, is a natural complex composite primarily consisting of three biopolymers: cellulose, hemicelluloses, and lignin. Lignocellulose typically contains cellulose (about 35-50 wt. %), hemicellulose (about 15-35%), and lignin (about 5-30%), depending on its origin. Natural cellulose molecules occur in elementary cellulose fibrils closely associated with other structural polysaccharides, such as hemicellulose, lignin, and pectin.
Efficient, cost-competitive production of fermentable sugars from recalcitrant biomass remains the largest obstacle to emerging cellulosic ethanol biorefineries. Biomass saccharification via biological conversion involves two key steps: lignocellulose pretreatment or fractionation followed by enzymatic cellulose (and perhaps hemicellulose) hydrolysis to produce fermentable sugars. The high processing costs of such a conversion process and the narrow margin between feedstock costs and sugar prices are the key obstacles for commercialization.
One of the most important technological challenges is to overcome the recalcitrance of natural lignocellulosic materials to allow for enzymatic hydrolysis to produce fermentable sugars. Lignocellulose pretreatment is perhaps the most costly step. Some estimates place the cost at about 40% of the total processing costs in addition, the recalcitrance impacts the cost of most other operations involving lignocellulose decomposition, including the reduction in lignocellulose size prior to pretreatment. Pretreatment of lignocellulosic materials is thus strongly associated with downstream costs, including enzymatic hydrolysis rate, enzyme loading, mixing power consumption, product concentration, detoxification if inhibitors are generated, product purification, power generation, waste treatment demands, and other process variables.
The recalcitrance of cellulosic biomass to enzymes is believed to be attributed to 1) the complicated linkages among several main polysaccharides, including cellulose, hemicellulose, and lignin, which restrict hydrolysis actions of cellulases, hemicellulases, and laccases; and 2) the inherent properties of cellulosic material, which include low substrate accessibility for cellulases, a high degree of polymerization (DP), and poor solubility of cellulose fragments in water. Pretreatment of lignocellulosic materials has thus been recognized as an important step in improving overall yield of products from such materials.
All lignocellulose treatments can be divided into for main categories: 1) physical methods, including dry milling (chipping, ball milling, and comminuting), wet milling, irradiation, microwave, and swelling reagents (e.g., ZaCl2); 2) chemical methods, including dilute acids (e.g., dilute H2SO4, H3PO4, HCl, acetic acid, formic acid/HCl), alkalis (e.g., NaOH, lime, ammonia, amine), organosolv, oxidizing agents (e.g., O3, NO, H2O2, NaClO2), cellulose solvents (e.g., cadoxen), DMAc/LiCl, and concentrated H2SO4; 3) physio-chemical methods, including, steam explosion with or without catalysts, CO2 explosion, ammonia fiber explosion or expansion (AFEX), hot water with flow-through, supercritical fluid extractions (e.g., CO2, CO2/H2O, CO2/SO2, NH3, H2O); and 4) biological methods (e.g., white rod fungi).
Recently, a Biomass Refining Consortium for Applied Fundamentals and Innovation (CAFI) undertook the first coordinated project to develop comparative information on the performance of leading pretreatment options. The consortium concluded that the best pretreatments included: dilute acid, flow-through pretreatment, ammonia fiber explosion, ammonia recycle percolation (ARP), and lime pretreatment. Additionally, two other possible pretreatments have been intensively investigated in Europe and Canada: steam explosion with or without SO2 impregnation, and organosolv. Typical conditions for biomass pretreatment are presented in Table 1.
TABLE 1Technologies and representative reaction conditions for lignocellulosicpretreatmentTemper-Pressure,Pretreatmentature,atmReactiontechnologyChemicals used(° C.)absolutetimes, minDilute sulfuric0.5-3.0% sulfuric130-200 3-15 2-30acid:acidco-currentFlowthrough0.0-0.1% sulfuric190-20020-2412-24pretreatmentacidpH-controlledwater or stillage160-190 6-1410-30waterpretreatmentAFEX100% (1:1)70-9015-20  <5anhydrousammoniaARP10-15 wt. %150-170 9-1710-20ammoniaLime0.05-0.15 g 70-1301-6 1-6 hourCa(OH)2/gbiomass
Although intensive lignocellulose pretreatment efforts have been made during the past several decades, current leading technologies, including dilute acid, SO2, controlled pH, AFEX, ARP, flow-through, organosolv, and lime pretreatment, have not yet been commercialized on a large scale due to high processing costs and great investment risks. But nearly all intensively-studied pretreatments share one or several of the common shortcomings: 1) severe pretreatment conditions (except APEX), resulting in degradation of sugars and formation of inhibitors; 2) low or modest cellulose digestibility because of the presence of residual lignin and hemicellulose; 3) high cellulase loading requirement; 4) slow hydrolysis rate because a significant fraction of pretreated lignocellulose remains crystalline; 5) large utility/energy consumption; 6) huge capital investment due to economy of scale; and 7) less co-utilization of other major components of lignocellulose except organosolv.
Dilute acid pretreatment (DA), typically using (dilute) sulfuric acid, is the most investigated pretreatment method. Conducted at relatively high temperatures (e.g., 150-200° C.) and pressures (e.g., 120-200 psia), DA pretreatment solubilizes acid-labile hemicellulose and thereby disrupts the lignocellulosic composite linked by covalent bonds, hydrogen bonds, and van der Waals forces. As a result, the condensed lignin remains on the surface of crystalline cellulose following DA, potentially hindering subsequent enzymatic hydrolysis.
Organosolv pulping offers environmentally benign benefits, smaller capital investment, co-product utilization, and lower feedstock transportation costs, as compared with Kraft pulping. Organosolv pretreatment has been developed from organosolv pulping, and was being studied for producing fermentable sugars after enzymatic hydrolysis as early as the 1980s. In general, organosolv pretreatments use a lignin-extracting solvent blend containing catalysts such as acids or alkalis, and water/organic solvents (e.g., ethanol and methanol) to extract lignin in high temperature and high pressure digesters.
Currently, the Lignol process is being developed as part of a commercial lignocellulose biorefinery in Canada. In that process, the lignin extracting step is carried out at about 180-200° C. and about 400 psi by a blend of ethanol/water in the range of about 50:50 (w/w) plus about 1% H2SO4 for 30-90 minutes. After organosolv pretreatment, a black liquor containing sulfur-free lignin, furfural, hemicellulose sugars, and other natural chemicals such as acetic acid, is further processed to: 1) precipitate and recover the lignin by diluting the black liquor with an aqueous steam, followed by filtering, washing, and drying; 2) recover and recycle the ethanol by flashing of the hot black liquor and condensation of the vapors, and distill the filtrate and washings from the lignin precipitation; 3) recover the acetic acid, furfural, and extractives from the distillation column, and separate xylose from the stillage; and 4) convert the hemicellulose oligosaccharides into sugars that can be fomented to produce more ethanol or other high value products. An economic analysis report by the Lignol Innovations Co. suggests that revenues from the multiple co-products, particularly the lignin, ethanol, and xylose fractions, ensure excellent economy for a small plant (about 100 metric tons per day), which is a twentieth of the input of a typical lignocellulose biorefinery.
Lignocellulose saccharification by concentrated acids is another common pretreatment method. Dissolving and hydrolyzing native cellulose in concentrated sulfuric acid, followed by a dilution with water was reported in the literature as early as 1883. Industrial wood saccharification involves many technical and economic problems, e.g., acid-resistant equipment, acid recovery, and final sugar yields. These problems have not yet been solved in spite of the numerous commercial processes that have been developed in Germany, Switzerland, Japan, the USA, and the former USSR since the beginning of the last century. The commercially tested technologies are the Scholler-Tornesch process in 1926, applying dilute sulfuric acid (0.4% H2SO4); the Bergius-Rheinau process in 1937, applying the super-concentrated hydrochloric acid (41% HCl); and the concentrated sulfuric acid process developed in 1948, using a membrane to separate sugar and acid. In the United States, the Madison process was developed during World War II as a continuous, rather than discontinuous, system based on the principle of the Scholler-Torneshch process. The processes using concentrated acids have the advantage of low reaction temperature, but costs for the corrosion-resistant equipment are very high. The main technical problems in applying concentrated sulfuric acid are soluble sugar/solid acid separation, acid recovery, and acid re-concentration.
Recently, a process called cellulose solvent- and organic solvent-based lignocellulose fractionation (COSLIF) was developed. A cellulose solvent (e.g., concentrated phosphoric acid or ionic liquid) enables the crystalline structure of cellulose to be disrupted. This type of pretreatment can also be carried out at low temperatures (e.g., at about 50° C.) and at atmospheric pressure, which minimizes sugar degradation. Subsequent washing steps are used to fractionate biomass; a first washing with an organic solvent to remove lignin; and a second washing, with water to remove fragments of partially-hydrolyzed hemicellulose (and potentially cellulose). The COSLIF approach produces highly reactive amorphous cellulose, which can be enzymatically hydrolyzed quickly and at high glucan digestibility yield
COSLIF can be regarded a hybrid technology for cellulose solvent-based biomass pretreatment, concentrated acid saccharification, and organosolv. As compared to other cellulose solvent-based biomass pretreatment technologies, this new technology involves lignin removal technology and efficient solvent recycling. As compared to organosolv, this technology can be conducted at lower temperatures, hemicellulose degradation is minimized (minimizing furfural as a major product), the resulting amorphous cellulosic materials is more reactive than that from organosolv, and a different combination of solvents is used. Unlike concentrated acid saccharification, concentrated phosphoric acid is used for limited, hydrolysis, resulting in long-chain polysaccharides that are insoluble in the solvents. Therefore, the separation of sugar with concentrated phosphoric acid is a solid/liquid separation. But in the concentrated acid saccharification, sugar/acid separation is a liquid/liquid separation. COSLIF also differs from most biomass pretreatment technologies (e.g., diluted acid, AFEX, hot water, steam explosion, etc.) in that the COSLIF process can generate amorphous cellulose that can be hydrolyzed easily and quickly, and can be used to isolate lignocellulose components, such as lignin.
A leading technology for lignocellulose pretreatment is disclosed in international patent application number PCT/US2006/011411 (publication number WO 2007/111605), which is incorporated herein in its entirety by reference. In embodiments, that patent application teaches a method that includes: adding a first solvent to lignocellulosic material to dissolve cellulose and hemicellulose; adding a second solvent to precipitate amorphous cellulose and hemicellulose and to partially solubilize lignin; separating the cellulose and hemicellulose from the lignin; separating the hemicellulose from the cellulose; recovering the products; and recycling the first solvent and the second solvent. The method of that invention thus includes separating glucose-containing cellulose from mixed sugar-containing hemicellulose. It also includes multiple organic solvents for fractionating cellulose, lignocellulose, lignin, and acetic acid, as well as multiple mechanical or electromechanical devices for separating solids (e.g., cellulose) from liquids (e.g., organic solvents).